How to make a semi-realistic metalic texture/material
There are many ways to do this.  Here's one that requires very little work, but gives you very little control:

Either find a photograph on line that you like or create one of your own.  You can use it as the basic image texture and, by processing its colors, to create the other maps.  There are a lot of things you can do, but here is a simple example node group, using a texture I grabbed from textures.com. Note that I deliberately exaggerated the effects so they would be obvious. I also used an HDR from sIBL.

The basic idea is that you turn the image into a black and white image and use the black and white image as the map you want.  Here I simply used color ramps to enhance the contrast.

EDIT: (I forgot this paragraph) The first three photographs are quick renders of just the image, the image + fake roughness map, the image + roughness and bump maps.

A second approach is to do the same thing, but to use tools outside Blender to create the other maps.  Crazy Bump has been mentioned.  You can also use your favorite photo editing program.

A third approach is to create your own textures procedurally. It would take an

The second and third approach require that you climb learning curves, but in the long run, both give you a lot of control over your own maps.

Finally, some of the commercial texture sites offer all of the necessary maps and provide free versions of some of them as an enticement.